Other probiotics are microscopic fungi, including Saccharomyces yeasts. Some probiotic microorganisms are normally resident in the digestive tract, while others are not (Guillot, 2009). Yousefi and Karkoodi (2007) studied the effect of the addition of different levels of probiotic (0.05, 0.1 and 0.15%) and Saccharomyces cerevisiae (0.05, 0.1 and 0.15%) to laying hens diet at 63 week of age.